1. Field of the Invention
The present invention relates to a current drive circuit for a current-drive display unit that uses organic electroluminescent elements (referred to as “EL elements” hereinafter), light emitting diodes (referred to as “LED elements” hereinafter) or the like that emits light by being supplied with current.
2. Description of the Related Art
Generally, a displaying operation of a display unit using EL elements or LED elements is controlled by a constant current drive circuit (a constant current driver). One conventional constant current drive circuit is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 2004-13053.
The constant current drive circuit of Japanese Patent Application Kokai No. 2004-13053 has a control voltage generating circuit section and a plurality of current output circuit sections for causing the display elements to emit light. The current output circuit sections are connected in parallel to the control voltage generating circuit section. Accordingly, a P-channel MOS transistor within the control voltage generating circuit section and a P-channel MOS transistor within each current output circuit section configure a current mirror circuit. Thus, constant current is generated from each current output circuit section.
In this constant current drive circuit, the source of the P-channel MOS (Metal Oxide Semiconductor) transistor in each current output circuit section is connected to a power-source pad via common wiring (power source wiring) and then to a power source potential from the power-source pad. Therefore, same power source potential is not supplied to the sources of the P-channel MOS transistors within the current output circuit sections because the voltage is decreased due to the resistance component(s) of the power source wiring. As a result, particularly in the current output circuit sections positioned away from the power-source pad, voltage VGS between the source and gate of the P-channel MOS transistor decreases, and thereby output current decreases.
Also, substrates of the P-channel MOS transistors within this constant current drive circuit are connected to the power-source pad via the shared wiring (power source wiring) and then to the power source potential from the power-source pad. Therefore, particularly in the current output circuit sections positioned away from the power-source pad, the potentials of the substrates of the P-channel MOS transistors decrease. Particularly in the current output circuit sections positioned away from the power-source pad, threshold voltages of the P-channel MOS transistors increase and the output currents decrease because of the substrate bias effect.
As described above, in the conventional constant current drive circuit, a value of the output current fluctuates in accordance with the position of the current output circuit section from the power-source pad. Hence, actually constant current cannot be generated with a high degree of accuracy.